System and method for electrorefining of silicon

ABSTRACT

The present disclosure provides methods and systems for electrorefining high-purity materials, for example, silicon. An exemplary system includes at least one cathode, an anode, and a reference electrode. At least one controller, for example a potentiostat, is used to control the potential difference between a reference electrode and a cathode or anode. The system can be operated in a single phase or multiple phase operation to produce high-purity materials, such as solar-grade silicon.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of PCT Application No. US2013/047775having an international filing date of Jun. 26, 2013, now published asInternational Publication No. WO 2014/004610 entitled “SYSTEM AND METHODFOR ELECTROREFINING OF SILICON”. PCT Application No. US2013/047775claims priority to U.S. Provisional Application No. 61/665,155 filed onJun. 27, 2012 and entitled “SYSTEM AND METHOD FOR ELECTROREFINING OFSILICON.” The entire contents of all the foregoing applications arehereby incorporated by reference.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under grant numberDE-SC0008862 awarded by the Department of Energy. The government hascertain rights in the invention.

TECHNICAL FIELD

The present disclosure generally relates to systems and methods forrefining silicon. More particularly, the present disclosure relates tosystems and methods of electrorefining high-purity silicon, for exampleusing a multi-electrode electrorefining apparatus.

BACKGROUND

Conventional refining of high-purity silicon is highly energy-intensive.For example, producing silicon of sufficiently high-purity for use insolar cells, semiconductor devices, and computer chips requires a veryhigh amount of electrical energy. Such high energy demand createssignificant costs, and in many instances, makes it prohibitivelyexpensive to refine sufficiently high-purity silicon.

For example, the Siemens method of producing high-purity siliconcomprises reducing trichlorosilane to polycrystalline silicon, whichoccurs by decomposing trichlorosilane on high-purity silicon rods orplates. This process is accomplished at greater than 1000° C., andutilizes water cooling to reduce the temperature of the reactor wall. Assuch, it requires a very high amount of electrical energy, andconsequently, significantly high energy costs.

Because of the high cost associated with producing sufficientlyhigh-quality silicon, there exists a need for systems and methods whichuse less electrical energy or otherwise offer additional advantages overprior approaches. Accordingly, systems and methods to refine silicon ina more energy efficient way, such as reducing the amount of electricalenergy required to refine silicon, are desired.

SUMMARY

The present disclosure provides improved systems and methods for therefining of high-purity materials, such as solar-grade silicon. In anembodiment, a system for electrorefining of silicon comprises an anode,a first cathode, and a reference electrode. Each of the anode, the firstcathode, and the reference electrode are coupleable to an electrolyte.The system further comprises a controller configured to control theelectrical potential between a reference electrode and at least one ofthe anode or the first cathode.

In another embodiment, a method for electrorefining of silicon comprisesproviding a system comprising an anode, a cathode, a referenceelectrode, an electrolyte, and a controller; applying an electricalpotential between the reference electrode and the anode to cause siliconto dissolve from the anode into the electrolyte; and applying anelectrical current between the anode and the cathode to cause silicon todeposit from the electrolyte onto the cathode.

In another embodiment, a method for electrorefining of silicon comprisesapplying a first electrical potential between a silicon-containing anodeand a reference electrode to cause silicon to dissolve from the anodeinto an electrolyte. The electrolyte couples the anode and the referenceelectrode. The method further comprises applying a first electricalcurrent between the anode and a first cathode to cause silicon todeposit from the electrolyte onto the first cathode; decoupling theanode from the electrolyte; coupling a second cathode to theelectrolyte; applying a second electrical potential between the secondcathode and the reference electrode to cause silicon to dissolve fromthe first cathode into the electrolyte; and applying a second electricalcurrent between the first cathode and the second cathode to causesilicon to deposit from the electrolyte onto the second cathode.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawing figures described herein are for illustration purposes onlyand are not intended to limit the scope of the present disclosure in anyway. The present disclosure will become more fully understood from thedetailed description and the accompanying drawing figures herein,wherein;

FIGS. 1A and 1B illustrate an exemplary electrorefining system inaccordance with an exemplary embodiment;

FIGS. 2A and 2B illustrate an exemplary electrorefining system inaccordance with an exemplary embodiment;

FIGS. 3A, 3B, and 3C illustrate an exemplary electrorefining system inaccordance with an exemplary embodiment;

FIGS. 4A, 4B, and 4C illustrate an exemplary electrorefining system inaccordance with an exemplary embodiment; and

FIG. 5 illustrates an exemplary electrorefining system in accordancewith an exemplary embodiment.

It will be appreciated that elements in the figures are illustrated forsimplicity and clarity and have not necessarily been drawn to scale. Forexample, the dimensions of some of the elements in the figures may beexaggerated relative to other elements to help to improve understandingof illustrated embodiments of the present disclosure.

DETAILED DESCRIPTION

The detailed description of various embodiments herein makes referenceto the accompanying drawing figures, which show various embodiments andimplementations thereof by way of illustration and best mode, and not oflimitation. While these embodiments are described in sufficient detailto enable those skilled in the art to practice the embodiments, itshould be understood that other embodiments may be realized and thatmechanical and other changes may be made without departing from thespirit and scope of the present disclosure.

Also, any reference to attached, fixed, connected or the like mayinclude permanent, removable, temporary, partial, full and/or any otherpossible attachment option. Additionally, though the various embodimentsdiscussed herein may be carried out in the context of electrorefining,for example electrorefining of silicon, it should be understood thatsystems and methods disclosed herein may be incorporated into othersystems to refine high-purity materials, for example silicon, inaccordance with principles of the present disclosure.

The various embodiments of an exemplary system, including at least oneanode, at least one cathode, at least one reference electrode, and atleast one system control comprise exemplary features hereinafterdescribed. The following description and the annexed drawing figures setforth in detail and demonstrate certain illustrative embodiments of thedisclosure. However, these embodiments are indicative of but a few ofthe various ways in which the principles disclosed herein may beemployed. Other objects, advantages and novel features will becomeapparent from the following detailed description when considered inconjunction with the figures.

Systems and methods in accordance with principles of the presentdisclosure provide the ability to refine certain high-purity materials,for example solar grade silicon. To assist in understanding the contextof the present disclosure, an exemplary high-purity siliconelectrorefining system 100 in accordance with the present disclosure isillustrated in FIGS. 1A and 1B. It will be appreciated that, whilevarious principles of the present disclosure are discussed herein withrespect to electrorefining of silicon, principles of the presentdisclosure may suitably be applied to electrorefining of various othermaterials, such as, for example, copper or aluminum.

In various embodiments, electrorefining system 100 comprises a vessel102. Vessel 102 can be configured to contain a number of electrodes,each of the electrodes in contact with an electrolyte 108. Vessel 102can comprise, for example, a metallic and/or non-metallic materialconfigured to facilitate an electrorefining operation. In variousembodiments, vessel 102 comprises a glassy carbon crucible; however, anysuitable material for vessel 102 may be utilized.

System 100 can further comprise an electrolyte 108. Electrolyte 108 cancomprise, for example, an electrically-conductive molten salt. Invarious embodiments, electrolyte 108 comprises a cation with a morenegative standard reduction potential than silicon, for examplepotassium, magnesium, calcium, sodium, barium, and/or lithium. However,any cation that is suitable for use as a component of an electrolyte inthe electrorefining of silicon is within the scope of the presentdisclosure.

In various exemplary embodiments, electrolyte 108 can further comprisean anion with an acceptably low cost and capable of forming a stableionic bond with a cation. For example, chlorine, fluorine, and oxygenare widely used in industrial electrorefining processes and can beprovided at a relatively low cost. In certain embodiments, chlorine andfluorine are preferable to oxygen as oxygen may form silicon dioxide onthe surface of electrodes, which may lead to reduced performance of theelectrode. However, any anion that is suitable for use as a component ofan electrolyte in the electrorefining of silicon is within the scope ofthe present disclosure.

Because higher operating temperatures of an electrorefining systemrequire higher electrical energy, and electrorefining systems operatemore effectively at lower operating temperatures, electrolyte 108 cancomprise, in various embodiments, a molten salt with a suitably lowmelting point. For example, a number of potential molten saltelectrolytes 108 comprise salts which melt at temperatures less than900° C., such as CaCl₂ (melting point 817° C.) and LiF (melting point848° C.). In various embodiments, electrolyte 108 comprises LiCl, whichhas a melting point of approximately 610° C., a cation (Li) with a morenegative standard reduction potential than silicon, and a relatively lowcost, non-oxygen anion (Cl). However, any molten salt with a suitablylow melting point that facilitates electrorefining of silicon is withinthe scope of the present disclosure.

In various embodiments, electrorefining system 100 further comprises atleast one cathode 110. Cathode 110 can comprise, for example, ahigh-purity silicon sheet or rod. In such configurations, silicondeposits on the surface of cathode 110 responsive to electrical energybeing applied to electrorefining system 100. In other embodiments,cathode 110 can comprise a non-silicon metal, such as tungsten. However,at higher operating temperatures (for example, at or greater than 600°C.), silicon can form silicides with metals such as tungsten. Therefore,in electrorefining systems operating at temperatures greater than 600°C., it is preferable for cathode 110 to comprise high-purity silicon. Invarious exemplary embodiments, systems and/or methods in accordance withprinciples of the present disclosure are operable over temperaturesranging from about 600° C. to about 1500° C.

Electrorefining system 100 can further comprise an anode 120. In variousembodiments, anode 120 can comprise a silicon-containing compound whichdissolves as electrical energy is applied to electrorefining system 100.In such configurations, electrical energy applied to electrorefiningsystem 100 causes silicon from the anode to dissolve into electrolyte108. The dissolved silicon is then free to travel throughout theelectrolyte.

Anode 120 can comprise, for example, a rod or sheet ofmetallurgical-grade silicon. In other embodiments, anode 120 comprisesan alloy, such as silicon and copper. However, any silicon-containingcompound which is capable of dissolving when electrical energy isapplied is within the scope of the present disclosure.

In various embodiments, electrorefining system 100 can further comprisea reference electrode 106. For example, reference electrode 106 cancomprise a relatively inert and/or stable material with a known standardreduction potential. In certain embodiments, reference electrode 106 iscomprised of at least one of glassy carbon or platinum. Referenceelectrode 106 can be configured to measure the potential differencebetween reference electrode 106 and either the cathode 110 or the anode120.

Electrorefining system 100 can further comprise a control system 104.Control system 104 can be configured, for example, to adjust theelectrical potential and/or current provided to electrorefining system100 (or a portion thereof) by a power source. In various embodiments,control system 104 comprises a potentiostat. In such embodiments,control system 104 can maintain the potential difference betweenreference electrode 106 and cathode 110 or anode 120, and can adjust thecurrent provided between cathode 110 and anode 120 to maintain a desiredelectrical potential. It will be appreciated that in various exemplaryembodiments, control system 104 is designed to control the voltagebetween a reference electrode and an anode (or cathode). In variousexemplary embodiments, minimal and/or no electrical current flowsthrough the reference electrode; rather, current flows between anode andcathode. Accordingly, in various exemplary embodiments, as the voltageapplied to the anode is controlled, the voltage on the cathode may beadjusted by control system 104 (e.g., a potentiostat) to allow theamount of current required by the voltage on the anode (and vice versa).

In an exemplary embodiment, electrorefining system 100 can be utilizedin a two-phase process for the electrorefining of high-purity silicon,for example solar-grade (i.e., 99.9999% pure or above) silicon. Duringoperation of electrorefining system 100, electrical potential is appliedto anode 120 through control system 104. As electrical potential isapplied, silicon is dissolved from anode 120 into electrolyte 108, alongwith one or more impurities. Responsive to electrical current flowingbetween anode 120 and cathode 110, silicon and other species dissolvedfrom anode 120 travel through electrolyte 108 and deposit on cathode110. Electrorefining system 100 can be configured to maintain a desiredrate of dissolution from anode 120 and rate of deposition on cathode110, for example by varying the potential difference applied betweenanode 120 and reference electrode 106. Alternatively, electrorefiningsystem 100 may be configured to vary and/or modify the rate ofdissolution from anode 120 and/or rate of deposition on cathode 110.

As illustrated in FIG. 1A, in certain exemplary embodiments, during thefirst phase of operation, control system 104 is used to maintain adesired potential difference between anode 120 and reference electrode106. For example, it may be desirable to maintain the potentialdifference between anode 120 and reference electrode 106 at or below thestandard oxidation potential of silicon at the operating temperature ofelectrorefining system 100. By maintaining a relatively low potentialdifference (for example, a potential difference below the oxidationpotential of silicon) between anode 120 and reference electrode 106, arelatively high rate of dissolution of silicon from the anode isachieved, but the concentration of one or more impurities in electrolyte108 is increased. After a sufficient amount of silicon has dissolvedfrom anode 120 and/or a sufficient amount of time has passed,electrorefining system 100 can be operated in a second phase ofoperation.

Turning now to FIG. 1B, in various exemplary embodiments, in connectionwith the second phase of operation of electrorefining system 100, anode120 is removed and second cathode 112 is inserted into vessel 102.During the second phase of operation, first cathode 110 is operated asan anode, and second cathode 112 is operated as a cathode. As electricalenergy is applied to electrorefining system 100 and/or portions thereof,control system 104 is used to maintain a desired potential differencebetween second cathode 112 and reference electrode 106. For example, itmay be desirable to maintain the potential difference between secondcathode 112 and reference electrode 106 between a minimum and maximumlevel. In various embodiments, the minimum potential difference is thestandard reduction potential of silicon at the process temperature. Invarious embodiments, the maximum potential difference is between about0.0 volts and 1.0 volts more negative than the reduction potential ofsilicon at the process temperature. By maintaining a relatively lowpotential difference, the amount of impurities deposited on secondcathode 112 by electrolyte 108 is maintained at a relatively low level.

In various embodiments, the maximum potential difference between secondcathode 112 and reference electrode 106 is limited by the targetconcentration of one or more impurities. As the potential differenceincreases, one or more impurities in electrolyte 108 are deposited morerapidly on second cathode 112, which can increase the concentration ofthe one or more impurities. Therefore, it may be desirable to maintain apotential difference between second cathode 112 and reference electrode106 between a minimum and maximum level, as described above. After asufficient amount of high-purity silicon has deposited on the surface ofsecond cathode 112 and/or a sufficient amount of time has passed, thesecond phase of operation of electrorefining system 100 can beterminated, and second cathode 112 removed for further processing.

In other exemplary embodiments, and with reference now to FIGS. 2A and2B, an electrorefining system 200 can be operated in a two-phase processto electrorefine high-purity material, for example, solar-grade silicon.FIG. 2A illustrates a first phase of operation of electrorefining system200. During the first phase of operation, the potential differencebetween a cathode 210 and a reference electrode 206 is controlled bycontrol system 204. As previously discussed in relation to the secondphase of operation of electrorefining system 100, the potentialdifference between cathode 210 and reference electrode 206 can bemaintained at a desired level to achieve a suitably high rate ofdeposition of silicon and a suitably low concentration of impurities onthe surface of cathode 210.

FIG. 2B illustrates a second phase of operation of electrorefiningsystem 200 in various exemplary embodiments. In connection with thesecond phase of operation, anode 220 is removed and second cathode 212is inserted into vessel 202. During the second phase of operation, firstcathode 210 is operated as an anode and second cathode 212 is operatedas a cathode. The potential difference between first cathode 210(operating as an anode) and reference electrode 206 is controlled bycontrol system 204. As previously discussed in relation to the firstphase of operation of electrorefining system 100, the potentialdifference between first cathode 210 (operating as an anode) andreference electrode 206 can be maintained at a desired level to achievea suitably high rate of dissolution of silicon and a suitably low rateof dissolution of impurities from first cathode 210 (operating as ananode).

Turning now to FIGS. 3A through 3C, in various embodiments, an exemplaryelectrorefining system 300 can be operated in a two-phase process toelectrorefine high-purity material, for example solar-grade silicon. Asillustrated in FIG. 3A, during a first phase of operation,electrorefining system 300 comprises a first cathode 310 and an anode320. During the first phase of operation, the electrical potentialbetween anode 320 and a reference electrode 306 is controlled by acontrol system 304. As discussed in relation to the first phase ofoperation of exemplary electrorefining system 100, the potentialdifference between anode 320 and reference electrode 306 can bemaintained at a desired level to achieve a suitably high rate ofdissolution of silicon and a suitably low rate of dissolution ofimpurities from anode 320.

In various exemplary embodiments, after a sufficient time of operationof electrorefining system 300 in the first phase configuration (forexample, after a time of operation of between about three hours andabout three days), as illustrated in FIG. 3B, anode 320 is removed andsecond cathode 312 is inserted into vessel 302. As illustrated in FIG.3C, during a second phase of operation, first cathode 310 is operated asan anode, and second cathode 312 is operated as a cathode. During thesecond phase, the potential difference between second cathode 312 andreference electrode 306 is controlled by control system 304. Aspreviously discussed in relation to the second phase of operation ofelectrorefining system 100, the potential difference between secondcathode 312 and reference electrode 306 can be maintained at a desiredlevel to achieve a suitably high rate of deposition of silicon and asuitably low concentration of impurities on the surface of secondcathode 312. Such a two-phase operation allows for the use of a singleelectrorefining system 300 to subject the silicon of anode 320 to twostages of dissolution and deposition, which can improve the quality ofsilicon ultimately deposited on the surface of second cathode 312 by,for example, minimizing the concentration of one or more impurities.

In other exemplary embodiments, and with reference now to FIGS. 4Athrough 4C, an exemplary electrorefining system 400 can be operated in atwo-phase process to electrorefine high-purity material, for examplesolar-grade silicon. As illustrated in FIG. 4A, during a first phase ofoperation, electrorefining system 400 comprises a first cathode 410 andan anode 420. During the first phase of operation, the potentialdifference between first cathode 410 and a reference electrode 406 iscontrolled by a control system 404. As discussed in relation to thesecond phase of operation of exemplary electrorefining system 100, thepotential difference between first cathode 410 and reference electrode406 can be maintained at a desired level to achieve a suitably high rateof deposition of silicon and a suitably low rate of deposition ofimpurities on the surface of first cathode 410.

In various exemplary embodiments, after a sufficient time of operationof electrorefining system 400 in the first phase configuration, asillustrated in FIG. 4B, anode 420 is removed and second cathode 412 isinserted into vessel 402. As illustrated in FIG. 4C, during a secondphase of operation, first cathode 410 is operated as an anode and thepotential difference between first cathode 410 (operating as an anode)and reference electrode 406 is controlled by control system 404. Aspreviously discussed in relation to the first phase of operation ofelectrorefining system 100, the potential difference between firstcathode 410 (operating as an anode) and reference electrode 406 can bemaintained at a desired level to achieve a suitably high rate ofdissolution of silicon and a suitably low dissolution rate of impuritiesfrom first cathode 410 (operating as an anode). Such a two-phaseoperation allows for the use of a single electrorefining system 400 tosubject the silicon of anode 420 to two stages of dissolution anddeposition, which can improve the quality of silicon ultimatelydeposited on the surface of second cathode 412 by, for example,minimizing the concentration of one or more impurities.

In various exemplary embodiments and with reference now to FIG. 5, anelectrorefining system 500 comprising a first control system 504 and asecond control system 505 can be operated in a single-phase process forelectrorefining of high-purity silicon. In such configurations, firstcontrol system 504 can comprise a potentiostat configured to maintain apotential difference between a first reference electrode 506 and acathode 510.

Second control system 505 can comprise, for example, a potentiostatconfigured to maintain a potential difference between a second referenceelectrode 507 and anode 520. In an exemplary embodiment, second controlsystem 505 maintains the electrical potential between second referenceelectrode 507 and the electrode not controlled by first control system504.

In various embodiments, electrorefining system 500 further comprises apartition 530. For example, partition 530 can comprise a non-conductivematerial which separates an electrolyte 508 into a cathode segment 534and an anode segment 536. In such embodiments, partition 530 maintainsphysical isolation between cathode segment 534 and anode segment 536.

Electrorefining system 500 can further comprise a counter electrode 532and a molten alloy 509. In such embodiments, a portion of counterelectrode 532 is in contact with molten alloy 509. A surface of moltenalloy 509 can be in contact with a surface of electrolyte 508 of cathodesegment 534 and anode segment 536. In such configurations, molten alloy509 operates as a cathode for anode segment 536 and as an anode forcathode segment 534.

In various exemplary electrorefining processes, electrorefining system500 is operated such that a first electrical current flows between anode520 and counter electrode 532, and a second electrical current flowsbetween cathode 510 and counter electrode 532. In an exemplaryembodiment, control system 504 is configured to maintain a desiredpotential difference between cathode 510 and reference electrode 506.Such a desired potential difference can comprise a potential differencewhich achieves a suitably high rate of deposition of silicon and asuitably low rate of deposition of impurities on the surface of cathode510. In such embodiments, control system 505 is configured to maintain adesired potential difference between anode 520 and reference electrode507. Such a desired potential difference can comprise a potentialdifference which achieves a suitably high rate of dissolution of siliconand a suitably low rate of dissolution of impurities from anode 520.Separately controlling these two potential differences can be beneficialby, for example, achieving low impurity concentrations in the productionof high-purity silicon.

Thus, the various systems and methods of electrorefining of the presentdisclosure provide means to produce sufficiently high-purity materials,for example silicon. The improved control of operating conditions, suchas operating temperatures and various potential differences,beneficially increases the purity of electrorefined materials, forexample solar-grade silicon.

The present disclosure has been described above with reference to anumber of exemplary embodiments. It should be appreciated that theparticular embodiments shown and described herein are illustrative ofinventive principles and its best mode and are not intended to limit inany way the scope of the present disclosure. Those skilled in the arthaving read this disclosure will recognize that changes andmodifications may be made to the exemplary embodiments without departingfrom the scope of the present disclosure. Various aspects andembodiments of the present disclosure may be applied to fields of useother than electrorefining of silicon. Although certain aspects of thepresent disclosure are described herein in terms of exemplaryembodiments, such aspects may be achieved through any number of suitablemeans now known or hereafter devised. Accordingly, these and otherchanges or modifications are intended to be included within the scope ofthe present disclosure.

While steps outlined herein represent exemplary embodiments ofprinciples of the present disclosure, practitioners will appreciate thatthere are other steps that may be applied to create similar results. Thesteps are presented for the sake of explanation only and are notintended to limit the scope of the present disclosure in any way.Benefits, other advantages, and solutions to problems have beendescribed herein with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any element(s) that maycause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as critical, required, or essentialfeatures or elements of any or all of the claims.

In the detailed description herein, references to “various embodiments”,“one embodiment”, “an embodiment”, “an example embodiment”, “anexemplary embodiment” etc., indicate that the embodiment(s) describedmay include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to utilize such feature, structure, or characteristicin connection with other embodiments if suitable, whether or notexplicitly described. After reading the description, it will be apparentto one skilled in the relevant art(s) how to implement principles of thedisclosure in alternative embodiments.

It should be understood that the detailed description and specificexamples, indicating exemplary embodiments, are given for purposes ofillustration only and not as limitations. Many changes and modificationsmay be made without departing from the spirit thereof, and principles ofthe present disclosure include all such modifications. Correspondingstructures, materials, acts, and equivalents of all elements areintended to include any structure, material, or acts for performing thefunctions in combination with other elements. Reference to an element inthe singular is not intended to mean “one and only one” unlessexplicitly so stated, but rather “one or more.” Moreover, when a phrasesimilar to “at least one of A, B, or C” or “at least one of A, B, and C”is used in the claims or the specification, the phrase is intended tomean any of the following: (1) at least one of A; (2) at least one of B;(3) at least one of C; (4) at least one of A and at least one of B; (5)at least one of B and at least one of C; (6) at least one of A and atleast one of C; or (7) at least one of A, at least one of B, and atleast one of C.

The invention claimed is:
 1. A method for electrorefining of silicon,the method comprising: applying a first electrical potential to asilicon-containing anode with respect to a reference electrode and afirst electrical current between the anode and a first cathode to causesilicon to dissolve from the anode into a molten electrolyte and tocause silicon to deposit from the molten electrolyte onto the firstcathode, the molten electrolyte coupling the anode, the first cathode,and the reference electrode; decoupling the anode from the moltenelectrolyte; coupling a second cathode to the molten electrolyte; andapplying a second electrical potential to the second cathode withrespect to the reference electrode and a second electrical currentbetween the first cathode and the second cathode to cause silicon todissolve from the first cathode into the molten electrolyte to causesilicon to deposit from the molten electrolyte directly onto the secondcathode.
 2. The method of claim 1, wherein the first electricalpotential and the second electrical potential are different.
 3. Themethod of claim 1, wherein the second electrical potential is between:(i) the reduction potential of silicon at a process temperature, and(ii) 1.0 volts more negative than the reduction potential of silicon atthe process temperature.
 4. The method of claim 3, wherein the processtemperature is between 600 degrees Celsius and 1500 degrees Celsius. 5.The method of claim 1, wherein the silicon deposited onto the secondcathode has purity in excess of 99,9999%.
 6. The method of claim 1,wherein the silicon deposited onto the second cathode has a higherpurity than the silicon deposited on the first cathode.
 7. The method ofclaim 1, wherein the electrolyte comprises at least one of calciumchloride (CaCl₂), lithium flouride (LiF), or lithium chloride (LiCl). 8.The method of claim 1, wherein at least one of the first electricalpotential or the second electrical potential is controlled by apotentiostat.
 9. The method of claim 1, wherein the anode comprisesmetallurgical grade silicon.
 10. The method of claim 1, wherein thereference electrode comprises at least one of glassy carbon or platinum.11. The method of claim 1, wherein the applying the first electricalpotential extends for a period of time between three hours and threedays.